This invention relates to streamer handling apparatus for use on seismic survey vessels, and is more particularly concerned with such apparatus for use on seismic survey vessels of the kind used in performing 3D marine seismic surveys covering large areas.
In order to perform a 3D marine seismic survey, a plurality of seismic streamers, each typically several thousand meters long and containing arrays of hydrophones and associated electronic equipment distributed along its length, are towed at about 5 knots behind a seismic survey vessel, which also tows one or more seismic sources, typically air guns. Acoustic signals produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones in the streamers, digitised and then transmitted to the seismic survey vessel, where they are recorded and at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed.
At the present time, a typical streamer array used by the Applicant comprises a 700 meter wide array of eight evenly spaced streamers, each about 4000 meters long. The streamers are towed by their lead-ins, ie the armoured electrical cables that convey electrical power, control and data signals between the vessel and the streamers, as described in the Applicant""s U.S. Pat. No 4 798 156, and their spread is controlled and maintained by MONOWING deflectors of the kind described in the Applicant""s U.S. Pat. No 5 357 892.
An array of this relatively large size allows 3D seismic surveys of large areas to be performed very efficiently. However, because the cost of performing such surveys is so high, there is continuous pressure to improve efficiency still further. One way of achieving this is to use even wider streamer arrays, containing even more streamers. However, the deployment and recovery of the streamers at the beginning and end of such a survey is itself a time-consuming process, which can also be hazardous. Each streamer is typically made up of 100 meter sections, and has auxiliary equipment such as depth controllers (xe2x80x9cbirdsxe2x80x9d), acoustic position sensors and the like secured to it at intervals along its length. This auxiliary equipment is typically attached to and detached from the streamer during deployment and recovery of the streamer.
It is therefore an object of the present invention to provide streamer handling apparatus which facilitates safe and speedy deployment and recovery of a streamer.
According to the present invention, therefore, there is provided streamer handling apparatus for use on a seismic survey vessel, the apparatus comprising:
a beam;
means for mounting the beam above normal head height above the streamer deck of the vessel, such that the beam extends generally longitudinally of the vessel, the mounting means including a pivotal connection to the sternmost end of the beam permitting said end to pivot about a horizontal axis extending generally transversely of the beam, and support means for lowering and raising the other end of the beam towards and away from the streamer deck; and
pulley means for supporting and guiding the streamer, said pulley means being secured to and movable along the beam.
In a preferred embodiment of the invention, the pulley means comprises carriage means movable along the beam, a pulley device, and means for suspending the pulley device beneath the carriage means.
Preferably, the suspension means comprises a collar which is rotatably supported on the carriage means and which is rotatable about the longitudinal axis of the beam.
Additionally, the pulley device is preferably pivotable about a horizontal axis which extends generally transversely of the beam.
The carriage means preferably includes at least one hydraulic motor arranged to move it in both directions along the beam, and is advantageously provided with hoisting means at its stem most end.
Conveniently, the support means comprises a telescopic member which is pivotally connected at one end to the mounting means and at the other end to said other end of the beam, and is hydraulically operated.
The apparatus may further comprise downwardly projecting guidance means which is positioned at said other end of the beam and which engages and pushes down the streamer when said other end of the beam is lowered.
The invention also includes a seismic survey vessel incorporating streamer handling apparatus as defined in any of the preceding statements of invention.